1. Field of the Invention
The present invention relates to an optical pickup device used for reading information of an optical disc in a DVD recorder, a personal computer, and the like.
2. Description of the Related Art
Generally, in an optical pickup device used in DVD recorder and the like, a laser light emitted from a light source is converted to a parallel light by a collimator lens, and the laser light transmitted through the collimator lens is collected on a disc surface of an optical disc by an objective lens to form an optical spot on the disc surface. In this case, when astigmatism (AS) occurs in the optical spot, the proportion of jitter in a detection signal obtained by receiving the reflected light increases, thereby adversely affecting the reproduction performance.
FIG. 10 is a view describing astigmatism of the optical spot on the disc surface of the optical disc. Reference numeral 50 is a groove, 51 and 52 are lands, 53 is a pit formed in the groove 50, and SP1 is an optical spot for reading the pit 53. FIG. 10A shows a state in which astigmatism is not occurring in the optical spot, where the optical spot SP1 is substantially a perfect circle. FIGS. 10B and 10C show a state in which astigmatism is occurring in the optical spot, where the optical spot SP2 is deformed to be vertically long in FIG. 10B, and the optical spot SP3 is deformed to be horizontally long (so-called cross-shaped AS) in FIG. 10C.
FIG. 9 is a view showing one example of a relationship between the jitter and the astigmatism in a BD (Blu-ray Disc; registered trademark). The horizontal axis indicates the amount of astigmatism of the optical spot on the disc surface, and the vertical axis indicates the proportion of the jitter in the detection signal. As the astigmatism becomes greater towards the positive side, the optical spot deforms to be vertically long as shown in FIG. 10B, and as the astigmatism becomes greater towards the negative side, the optical spot deforms to be horizontally long as shown in FIG. 10C. The shaded area of FIG. 9 shows the margin region of the amount of astigmatism when a tolerable range of 1% is set with respect to a bottom jitter (a minimum value of jitter). Since the value of the bottom jitter is about 6%, 7% is the upper limit value, and the margin of the amount of astigmatism is in the range of −40 [λm] to +10[λm].
According to FIG. 9, the amount of astigmatism when the jitter becomes a minimum is not 0, but is slightly shifted to the negative side from 0. That is, the jitter becomes a minimum not when the optical spot is a perfect circle, but when slightly horizontally long. This tendency is the same in CD (Compact Disc) and DVD (Digital Versatile Disc) (it should be noted that the amount of astigmatism when the jitter becomes minimum differs depending on the medium). The factor for the amount of astigmatism corresponding to the bottom jitter to shift towards the negative side includes interference (intersymbol interference) with respect to pits adjacent in a track direction being less likely to occur when the optical spot becomes horizontally long. Therefore, the reproduction performance can be enhanced by correcting the amount of astigmatism to an optimum value at which the jitter becomes a minimum.
One of the methods for correcting the astigmatism includes a method of using liquid crystal element. In this method, the liquid crystal element is arranged on a light path between a light source and an objective lens, and an electrode of a predetermined pattern for correcting the astigmatism is arranged on each of a pair of substrates of the liquid crystal element. The orientation direction of the liquid crystal molecule of the portion sandwiched by the electrodes is changed by controlling the voltage to be applied to the electrodes, and the astigmatism of the optical spot is corrected by providing a phase difference to the light transmitting through such portion. According to such method, however, the electrode pattern that corresponds to each wavelength must be formed when reading the medium corresponding to a plurality of wavelengths such as BD, DVD, and CD with one optical pickup, whereby the number of layers of the electrode pattern becomes large, and the cost of the liquid crystal element itself becomes high.
In place of the method using the liquid crystal element described above, a method of correcting the astigmatism by adjusting the tilt of the collimator lens may be adopted. In this method, the collimator lens is fixed through adhesion while being tilted by an optimum angle (at which jitter becomes a minimum) in advance in time of adjustment. However, according to this method, since the collimator lens is fixed, the astigmatism cannot be corrected for each light source if light sources corresponding to a plurality of wavelengths are provided. For example, when the tilt of the collimator lens is adjusted with respect to the light from the BD light source, the jitter can be minimized with respect to the BD but the jitter cannot be minimized with respect to the DVD and the CD since the amount of astigmatism at which the jitter becomes a minimum differs depending on the medium.
The optical disc also has a problem of spherical aberration in addition to astigmatism described above. For example, when reproducing a medium having two recording layers, the spherical aberration of the second recording layer becomes large if adjusting the spherical aberration focusing on the first recording layer, and the spherical aberration of the first recording layer becomes large if adjusting the spherical aberration focusing on the second recording layer, whereby the reproduction performance lowers. Therefore, a correction that minimizes the spherical aberration in each layer is required. For such correction, a method of moving the collimator lens in an optical axis direction is disclosed in Japanese Unexamined Patent Publication No. 2005-235269 and Japanese Unexamined Patent Publication No. 2005-209268. However, the spherical aberration and the astigmatism cannot be simultaneously corrected by simply moving the collimator lens in the optical axis direction.